One of the usages of communication networks, such as cable television networks and the Internet, is provision of motion pictures in the form of video streams. Video streams are formed from a sequence of frames, which are still pictures shown at a specific instance. The transmission of video streams requires large amounts of bandwidth and therefore compression methods are generally applied to video streams before transmission. For a given quality level, some video frames (e.g., frames having a large amount of detail) can be compressed only to a small extent, while other video frames (e.g., frames merely showing a blue sky) can be compressed to a large extent. Therefore, many video compression methods provide variable bit-rate (VBR) video streams, in which different frames are compressed by different amounts.
Some communication networks and storage devices, however, are adapted to handle constant bit rate (CBR) video streams, which at all times include the same amount of data bytes. Therefore, most communication networks are required to choose between the ease of handling of CBR streams and the quality smoothness of VBR streams.
U.S. Pat. No. 5,734,432 to Netravali et al., issued Mar. 31, 1998, the disclosure of which is incorporated herein by reference, describes transmitting auxiliary signals not related to a main transmission on a channel, when a main video signal transmitted on the channel does not require all the bandwidth of the channel.
U.S. Pat. No. 5,966,120 to Arazi et al., issued Jan. 20, 2004, the disclosure of which is incorporated herein by reference, describes adding auxiliary data during transmission of one or more VBR streams to form a constant bit rate transmission. The auxiliary data includes non real time information, representing additional content beyond that included in the VBR stream. The additional content may include video clips, such as advertisements, for insertion in the video of the VBR stream and/or non-video information, such as advertisements for printing by a computer.
The above patents reduce the waste of bandwidth on the transmission channel due to the varying size of the VBR stream, by transmitting additional content on the available bandwidth. Not always, however, is there available content which needs to be transmitted with the VBR stream. In addition, the multiplexing of the additional content with the VBR stream and its separation at the destination may be a complex procedure.
Attempts have been made to achieve the ease of handling of CBR streams, with a quality level close to that of a VBR stream.
PCT patent publication WO 2006/099086, published Sep. 21, 2006 and assigned to Qualcomm Incorporated, the disclosure of which is incorporated herein by reference, describes a compression method which uses an adaptive quantization parameter, to provide a VBR quality compression (i.e., a compression with a substantially constant quality) which fits into a CBR environment. The compression method prescribes that the number of frames represented by a predetermined block size of the compressed video stream is varied over time, according to the achievable compression ratio. This method requires using a relatively large buffer which incurs a substantial delay, but does not achieve a completely smooth quality video stream. In addition, the resultant video stream is in a format not supported by some standard consumer electronic devices.
PCT patent publication WO 2005/109895, published Nov. 17, 2005 and assigned to Konin-Klijke Philips Electronics N.V., the disclosure of which is incorporated herein by reference, describes a compression method in which a low quality VBR stream is generated as a base of a compressed stream. Enhancement information is added to the VBR stream, where the compression results in a reduced amount of data, in a manner which forms a CBR stream. The resulting stream has a variable quality, having a higher quality when the VBR compression leaves much room for enhancement information and a lower quality at times when the VBR compression requires the entire available bandwidth.